<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
</head>
<body>
    <div id="app">
        <!-- <p>{{count}}</p>
        <p>{{aplus}}</p> -->
        <p>{{a.b}}</p>
        <button @click="change">change</button>
    </div>
    <script>
        const vm = new Vue({
            el:"#app",
            data () {
                return {
                    msg: 10,
                    b: 20,
                    a:{
                        b: 10
                    }
                }
            },
            methods: {
                change () {
                    this.$set(this.a, this.a.b, 500)
                }
            }
        })
    </script>
</body>
</html>